The present invention relates to apparatus for monitoring the distribution and mixing of chemical species in a region of space such as a vessel or a free space such as a space containing an engine exhaust plume.
The spatial and temporal distribution and mixing of chemical species can be a critical determinant of the performance of chemical reactors. For example, the combustion chamber of an internal combustion engine is a chemical reactor in which the spatial and temporal variation of the air-fuel ratio prior to ignition bas a significant influence on both fuel efficiency and emissions performance. Various attempts have been made to analyse the spatial variation of the air-fuel ratio within internal combustion engines. Typically the cylinders and/or pistons of internal combustion engines have been provided with windows through which it has been possible to take graphs using high-speed camera equipment so as to image variations in the air-fuel ratio. It is known for example to introduce a fluorescent dopant into fuel introduced into the cylinder so as to provide a visual indication of fuel distribution within the air-fuel mixture.
A wide range of tomography modalities have been developed which encompass the classes of hard-field, soft-field and emission techniques. Each of these classes has various strengths and weaknesses. In particular, all of the classes require an inversion calculation to reconstruct the distribution of a parameter of interest. The stimulation process for fluorescence tomography has a hard-field nature in that the only material which can be stimulated to fluoresce is that lying on the geometrical path of input radiation, whilst detection of the isotropically emitted fluorescence clearly has an emission nature.
Work has been conducted in the case of X-ray fluorescence as described by Cesareo R., and Mascarenhas S. (1989), A new tomographic device based on the detection of fluorescent X-rays, Nucl. Instr. Meth. A277, 669-672. This paper points out that collimation of both the stimulation beam and detector acceptance results in an unambiguous determination of the spatial region referred to as the xe2x80x9cspace-pointxe2x80x9d from which detected fluorescence photons were emitted. In the described X-ray fluorescence case however the material under study strongly attenuated both the stimulation beam and the fluorescence photons and this required an additional complexity in image reconstruction by de-convoluting the attenuation. Thus this earlier work appeared to indicate that using collimated stimulation beams and detector fields of view did not avoid the need for computationally intensive image reconstruction techniques.
The technique of planar laser-induced fluorescence (PLIF) has been the most successful to date in providing information on the mixing and combustion processes in internal combustion engines. To implement this technique, large glass inserts are placed between the cylinder head and the engine block. The laser emission is formed into a sheet, passing through the cylinder head and exciting fluorescence. The fluorescence is observed in the orthogonal direction through an elongated piston with a central glass window and a mirror, typically by a CCD camera However, PLIF systems can only produce a low number of frames per cycle because of the comparatively low frequency of the light samples, determined by the low repetition rate (10-100 Hz) of the laser sources.
The known systems require extensive optical access which in ton requires substantial modifications to an internal combustion engine the performance of which is to be assessed, The provision of relatively large optical windows in a cylinder wall for example can significantly affect engine performance as compared with an engine in which no such optical windows are provided. Furthermore, the temporal resolution of such known techniques is limited to a few Hertz due to the pulsed laser sources used. These techniques are not suitable for application to routine engine operations.
It is known that air containing hydrocarbons absorbs laser radiation to a greater extent than air fee of hydrocarbons if the laser radiation is at a frequency which excites vibrational/rotational transitions in hydrocarbon molecules. In particular, the presence of CH3, CH2 and CH groups in molecules in an air/hydrocarbon mixture results in a greater absorption due to various vibrational transitions and their overtones and combinations than is the case with air not containing such molecules.
It is an object of the present invention to exploit the specific absorption of radiation to monitor spatial and temporal variations in the air-fuel ratio within for example an internal combustion engine.
It is a firer object of the present invention to provide an improved apparatus for monitoring the spatial distribution within a vessel of a chemical species without requiring the use of computationally intensive techniques to produce a representation of the distribution.
According to the present invention, there is provided an apparatus for monitoring the distribution within a defined space of a chemical species, wherein a plurality of radiation sources and radiation detectors are distributed around the perimeter of the space, the radiation sources being distributed to emit beams of radiation across the interior of the space, the wavelength of the radiation being selected such that an interaction occurs between the radiation and the chemical species which can be detected by the detectors, and means being provided for deriving a representation of the distribution of the chemical species within the space from the detected interactions.
In one embodiment of the present invention a plurality of radiation sources and radiation detectors are distributed around the perimeter of the space such that radiation from each source is directed along a predetermined path towards at least one detector, the sources emit radiation at a wavelength selected to excite vibrational and/or rotational traditions in at least one of the chemical species such that radiation is absorbed to a greater degree by the said at least one species than by at least one other species, absorption of the radiation occurring along each of the predetermined paths is monitored to provide a measure of the path integral of the concentration of the said at least one species along each path, and a representation of the distribution of the concentration of the said at least one species within the space is derived from the measured path integrals of concentration.
The radiation sources and detectors may be arranged in pairs such that each detector receives radiation via a respective path from a respective source. Alternatively, at least one source may be arranged to direct radiation in a beam defining respective predetermined paths to each of a plurality of detectors, each detector having a collimated field of view which includes only the respective predetermined path.
In the case of the application of the invention to monitoring the spatial variation of air-fuel ratios within internal combustion engines, the specific if weak absorption of electromagnetic radiation in the near infra-red region of the electromagnetic spectrum (1 xcexcm to 2.5 xcexcm) may be exploited to distinguish between absorption resulting from overtones and combinations of various hydrocarbon vibrational and/or rotational transitions, particularly such transitions arising with CH3, CH2 and CH groups in molecules. A suitable wavelength for use in the apparatus of the first embodiment of the present invention is 1700 nm (xe2x88x9215, +50 nm), as hydrocarbons exhibit weak absorption at this wavelength. Thus in contrast to prior art techniques, which rely upon the addition of fluorescent dopants to the fuel, the first aspect of the present invention uses the inherent absorption properties of hydrocarbon systems to derive data described in the spatial variation of the air-fuel ratio without in any way modifying the chemical constituents by for example adding dopants to the fuel. Furthermore, although each source-detector pair can provide a measure of the path integral (or average) of the hydrocarbon concentration only along the path between that pair, the spatial location of that path can be accurately determined and, providing data is extracted from a sufficient number of paths, it is a relatively well known computational task to produce a representation of the distribution of the hydrocarbons using conventional tomographic techniques. The sources may be lasers, and for the engine application the sources should have a high modulation bandwidth, e.g. of the order of 40 kHz or more. Temporal resolutions as high as 20 kHz may thus be achieved, in contrast to the low temporal resolutions achievable with known techniques.
Preferably, each radiation source comprises means for directing radiation having a further wavelength along the predetermined path to the detector of the respective pair, the further wavelength being selected such that it does not excite vibrational and/or rotational transitions in any of the components, and mean are provided for comparing the absorption of the radiation of the two wavelengths to compensate for absorption which is not related to vibrational and/or rotational transitions excited in the said at least one component. A suitable wavelength for the further radiation source is 1550 nm in the case of long-chain saturated hydrocarbons being the species of interest
The radiation output of each detector may be time division multiplexed such that only one wavelength is emitted at any one time. Alternatively, frequency division multiplexing may be used, with the absorption of different simultaneously transmitted wavelengths being measured by de-multiplexing by the detector electronics.
In an alternative arrangement, the detectors comprise optical means for distinguishing between optical characteristics of the outputs of the sources which are of different wavelength. The outputs of different wavelength may be differentially polarised, with the optical means being polarisation sensitive, or the optical means may comprise a spectrometer.
Preferably at least one source comprises a tunable laser and means are provided for tuning the laser to take account of variations in the absorption characteristics of the space.
According to a second embodiment of the present invention, each radiation source produces a collimated radiation beam directed along a predetermined respective path to stimulate fluorescence in a chemical species, the detectors being responsive to fluorescence photons emitted by the chemical species, each detector having a collimated field of view which intersects a plurality of the predetermined paths, the apparatus further comprising means for identifying the position of the source of fluorescence photons detected by any detector by correlating the detection of fluorescent photons with the energisation of the sources, and means for deriving a representation of the distribution of the chemical species from the correlated fluorescence photon detection and position data
Preferably, means are provided for sequentially energising the sources such that only one beam intersected by the field of view of one detector is energised at a time. An alternative to sequential operation of the sources is to encode each source by intensity modulation at a given frequency, coupled with de-multiplexing of the detector signals.
Each radiation source may comprise a high-power laser, and may deliver a series of pulses of UV wavelength. Alternatively, the sources may be diode lasers, LEDs or may deliver radiation derived from a gas-filled discharge lamp. Each source may produce radiation with a wavelength in the range 200-500 nm
The sources may be arranged to generate beams which intersect in regions of the defined space, each intersection region also being intersected by the collimated field of view of at least one detector.
The second embodiment of the present invention makes it possible to determine the concentration of various chemical species in a liquid-phase or gas-phase mixture without requiring a computationally intensive inversion step. Because of the weak attenuation of the stimulation and fluorescence photons, a simple geometrical reconstruction of the true space-points can be used. It is necessary to know the fluorescence properties of the gas or liquid phase subject well in order to ensure that the detected fluorescence photons do indeed come from the target chemical species. It is also necessary to ensure that there can be no ambiguity of the space-point from which fluorescence photons are emitted, either by sequential operation of the sources or by means of encoding each source (for example using frequency encoding).